Integrate f(x, y, z) = (x 2 + z 2 )0.5 over the circle, r(t) = (acost)j + (asint)k, 0 ≤ t ≤ 2π
Question
Integrate f(x, y, z) = (x 2 + z 2 )0.5 over the circle, r(t) = (acost)j + (asint)k, 0 ≤ t ≤ 2π
Solution
The problem you've given is a line integral over a circle in the yz-plane. However, the function f(x, y, z) = (x^2 + z^2)^0.5 is a function of x, y, and z. The circle r(t) = (acos(t))j + (asin(t))k only provides y and z values, not x values.
If we assume x = 0, the function simplifies to f(x, y, z) = |z|, and the circle becomes r(t) = (acos(t))j + (asin(t))k = ajcos(t) + aksin(t).
The line integral of a scalar function f over a curve C parametrized by r(t), a ≤ t ≤ b, is given by ∫_C f ds = ∫ from a to b f(r(t)) |r'(t)| dt.
Here, r'(t) = -asin(t)j + acos(t)k. So, |r'(t)| = sqrt[(-asin(t))^2 + (acos(t))^2] = a.
The function f(r(t)) = |asin(t)| because z = asin(t) from the parametrization of the circle.
So, the line integral becomes ∫ from 0 to 2π |a*sin(t)| * a dt = a^2 ∫ from 0 to 2π |sin(t)| dt.
The integral of |sin(t)| from 0 to 2π is 2, so the line integral is 2a^2.
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